Method of manufacturing permanent magnets by continuous castings



June 22, 1965 A. l. LuTl-:IJN 3,189,957

METHOD OF MANUFACTURING PERMANENT MAGNETS BY CONTINUOUS GASTINGS Filed Feb. 27, 196s 2 sheets-sheet 1 June 22, 1965 Filed Feb. 2'?, 1963 Y BY CONTINUOUS CASTINGS 2 Sheets-Sheet 2 d my Tw Sec' .f5.1 as 7.3 10

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INVENTOR. ,7mm/f www@ United States Patent O 3,189,957 METHOD F MANUFACTURING PERMANENT MAGNETS BY CONTINUOUS CASTINGS Anthonie Izaak Luteijn, Emmasingel, Eindhoven, Netherlands, assignor to North American Philips Company,

Inc., New York, N.Y., a corporation of Delaware Filed Feb. 27, 1963, Ser. No. 261,255 Claims priority, application Netherlands, May 4, i956, 206,892.; Feb. 15, 1963, 239,045 3 Claims. (Cl. -22-200.1)

This application is a continuation-impart of my copending application Serial No. 656,374 filed May 1, 1957, now abandoned.

The invention relates to the manufacture of permanent m-agnets, particularly magnetically-anisotropic alnico-type permanent magnets having a directional-grain or crystal structure and consisting of an alloy of about to 42% cobalt, about 10 to 20% nickel, about 6 to 10% aluminium and the remainder principally iron. The alloy may also contain additional elements such as up to about 8% copper, up to about 4% niobium, up to about 8% tantalum wherein the niobium and/or tantalum may be partially replaced by titanium.

Since the discovery of alnico magnets in the early 1930s much work has been done toward improving the magnetic properties thereof, particularly the (BH)mx and also toward reducing the cost of manufacture. It is, of course, important that the advantages of improved magnetic properties are not oiiset by an increase in the cost of manufacture and vice-versa, and in evaluating advances in this art these two factors must be considered together. This is particularly the case for plug magnets for loud speakers which represents a large portion, perhaps 40%, of the production of alnico, because in many cases particularly for miniature transistor radio receivers, both small size and weight, which result from a high (BH)m ,X, as well `as low cost are necessary. The major part of alnico-magnet production has been, and at the present time still is being, carried out by batch casting in molds of various types, such as baked sand or shell molds. A

Perhaps the greatest advance in the alnico-magnet field was made by lonas (U.S. Patent 2,295,082) in the late 1930s in which magnet bodies of alnico alloys of certain compositions were subjected to a magnetic iield while in a temperature r-ange slightly below the Curie point to make the same magnetically anisotropic in a preferred direction of magnetization. While this treatment complicated and increased the cost of manufacture, the improvement in the (BH)mx values -obtained was so great (approximately 3-fold) as to oiset such increased cost and as a result this method came into extensive use, particularly for making loudspeaker magnets.

It was later disclosed, for example, in U.S. Patent 2,578,407, to further increase the (BH)max value of the magnetically-anisotropic magnets of Jonas by providing the molds with chill plates, insulators or both to thereby produce a directional-grain structure. However, in this method the cast magnet bodies were limited to length-todiameter ratios not greater than about 5 to 1, preferably not greater than 2 to 1. According to Patent 2,578,407 it was possible to cast magnet bodies about 1 inch in diameter and l inch long and obtain therefrom magneticallyanisotropic magnets with a (BH)max of slightly less than 3,l89,957 Patented June 22, 1965 ICC tion, to obtain directional-grain magnetically-anisotropic alnico magnets having a (BH)mX of about 6 to 7,. this could be done only by means of methods which were so expensive and complicated as to make such magnets cornmercially-impractical for most applications, particularly the important iield of loudspeaker plug magnets.

The main object of the invention is to overcome the above-mentionedI disadvantages of directional-grain methods and to manufacture in a simpler and less expensive manner directional-grain magnetically-anisotropic permanent magnets having magnetic properties which are at least as good as those of magnets produced by present-day mass production methods, for instance (BHLMx of at least 6 m.g.o.

A further object is to provide a method of manufacturing such magnets without waste of material i.e. with substantially utilization of the melt.

Another object of the invention is to provide a method of manufacturing elongated or rod-shaped magnet bodies whose magnetic properties are uniform throughout their length so that shorter magnet bodies cut therefrom will have uniform magnetic properties.

According to the method of the invention I continuously feed a melt of an alnico alloy of the above-composition to one end of a tubular shaped mold which defines a freezing or solidiiication zone and whose other end is closed by a support or starting rod and move the support axially in a predetermined manner, preferably with a step-wise movement, while withdrawing substantially all of the heat of solidification in the directionof movement to thereby progressively solidify successive portions of the melt while in the solidiiication zone and form a rod shaped magnet body with a uniform and intensive directional-grain structure.

In order to withdraw substantially all of the heat of solidiiication in the direction of movement I artificially cool the portion of the solidied rod-shaped body which extends from the mold, and in practice l iind it advantage# ous to supply controlled heat to the mold in the vicinity of the solidifcation zone.

The movement of the melt while in the solidiiication zone is very important, both as to speed and type of movement. When using a continuous movement the speed should be maintained substantially constant and at a predetermined rate which is less than about 0.1 mms] sec.

In accordance with a preferred embodiment of the method l employ a step-wise movement in which the length of the time periods during which the material remains stationary and the velocity during the moving periods are maintained within predetermined values which are correlated.

With both such movements high (BHlmax-values, for instance, more than 6 m.g.o. have been obtained in mass production.

Furthermore it can be remarked that the (BHLmX-` value depends upon the degree of axial crystal orientation which can be obtained in such a way that (BI-I)mx-values more than 6 106 g.o. are only obtainable with a high degree of crystal orientation. If no crystal orientation is present the (BH)maX-value is about 5 106 g.o. Between the last mentioned (BH)mX-value and the (BH)mx-value of 6 l0 g.o. there is a range in which partial crystal orientation occurs and for this reason the methods according to the invention might be applied as well.

Generally the rod-shaped bodies can be drawn from a tube of about 10 cm. long with a velocity of 0.1-10 mms./ sec. combined with stationary periods which vary between about 10 see-200 sec., the lower velocities being used with the shorter stationary periods. If these conditions are not followed, particularly the minimum stationary alsace? periods, and whenl using continuous movement the maximuc velocity, rods are obtained with poor surface conditions and often breakage occurs. Furthermore, there is hardly any crystal orientation and consequently the (BH),mx-value is less than5 106 g.o.

If for example a (BH)maX-value of about 8X106 g.o. is desired, a velocity during each step of 10 mms/sec. and stationary periods of 180 sec. can be used. If this (BH)mx-value is to be obtained with a velocity during each step of 5 mms/sec. then accordingly smaller stationary periods should be chosen, for example, between 30 and 60 sec. If, on the contrary, with the velocities as mentioned above, shorter stationary period are chosen, lower (BH),x-values are obtained. The same effect occurs if with the same stationary period the velocities are chosen higher. Y

In order that my invention may be. clearly understood and readily carried into effect l shall now describe the same in more detail in connection with the accompanying drawings in which:

FIG. 1 is a schematicv diagram of apparatus used in carrying out the invention,

FIG. v2 is an enlarged view of a portion of FIG. 1 and FIG. 3 is a graph showing relationship between velocity and time during which the material is stationary.

Referring to FIG. l, the material 1, which is the alloy from which the magnet is to be formed and is in a granular lform, is placed in a hopper 2 from which it is driven into a crucibley 4 by means of a screw 3 rotated at an adjustable speed by a suitable drive mechanism (not shown). Alloy 1 consists essentially of about 15 to 42% cobalt, about 10 to 20% nickel, about 6 to 10% aluminium, and the balance principally iron. In the specific example to be discussed I use an alloy of 24% cobalt, 14% nickel, 8% aluminium, 3% copper, J/z% niobium and remainder iron.

The Crucible 4 consists of a ceramic container Whose lower end 5 forms a tubular mould having a cross section the same as that of the elongated magnet body to be formed, in this case a rod-shaped body having a diameter of about 10 to 20 mms. Surrounding crucible l is a highfrequency coil 6 which is supplied from a suitable source (not'shown) and which serves to form a melt 7 from the alloy material 1. To prevent the melt from being attacked by gases, particularly oxygen, the upper portion of the Crucible is lled with an inert or non-oxidizing atmosphere, such as argon, helium, or hydrogen, which enters through an inlet opening 8 and leaves through an outlet 9. The crucible is jacketed with heat-insulating material 10.

A resistance heating coil 11, which assists in controlling the direction of Withdrawal of the heat of solidication, surrounds mould 5 and is supplied from a suitable source (not shown). The lower end of mould 5 is closed by a rod-shaped body 12 which may be of iron or of the same material as the melt 7. Body 12 is given a predetermined axially motion M by two wheels 13 which may be rotated at variable speeds with either a uniform or stepwise movement by means of suitable driving mechanisms which are well-known in the art.

`As shown in FIG. 2 the mould 5 denes a solidication zone 14. In order to withdraw substantially all of the heat of solidiication in an axial direction from the molten material in the solidication zone the rod-shaped body being formed passes through a water-cooled device 15 of suitable construction.

Bodies 16 having a length of several meters are cut from the rod being formed and are subjected to a magnetizing and heat-treating apparatus 17 in which they are made magnetically anisotropic in known manner. Bodies 16 are then cut into pieces of small length, for instance 10 to 20 mm. for loudspeaker magnets which small pieces are then finally magnetized in an axial direction; the above-mentioned heat-treatment may be applied to the small bodies rather than to bodies 16.

In the above described apparatus the rod-shaped magnet body 16, having a diameter of about 14 mm. and consisting of an alloy comprising 24% Co, 14% Ni, 8% Al, 3% Cu, 0.5% Nb, balance principally Fe, was given a continuous movement of 0.01 mm./sec. After the abovementioned heat-treatment-magnet bodies having a length of about 15 mm. separated, for instance by a saw from the rod-shaped body and magnetized axially, were found to have a (BI-Dm,X of 7.9 m.g.o. at a remanence of 13450 gauss and a coercive force of 770 oersted.

Similar rod-shaped magnet bodies of the Same type were given a continuous movement of about 0.04 mm./ sec. The final results were (Bl-Dm,x 6.7 m.g.o., remanence 13360 gauss and coercive force 740 oersted.

Preferred embodiments of the invention are obtained by using a discontinuous or stepwise movement of the rod-shaped magnet body, said embodiments being carried out with alloys of the specic composition as given above for the continuous movement. In FIG. 3 the velocity (d in mm./ sec.) is given on the vertical axis and the stationary periods (t in sec.) are given on the horizontal axis.

From FIG. 3, in which the results of the measurements relate to the (Bl-l)max value in m.g.o., it is seen that,

' with a constant value of the velocity of 10 mm./sec. the

(BI-l),max value increases from about y5 m.g.o. to 8 m.g.o. as the time of the stationary period increases from a value of 30 sec. to 180 sec. respectively, (coercive force 710-780 oersted, remanence 12500-13200 gauss). Furthermore, it can be seen that with a constant value of the time of the stationary period, of 30 sec., the (BH)mx increases from a value of about 5 m.g.o. to 8.8 m.g.o. (coercive force 710-790 oersted, remanence 12500-13600 gauss) as the velocity decreases from 10 nim/sec. to 2 ram/scc. respectively. Generally it can thus be derived that for every preselected (BH)maX value obtainable, e.g. lying between 8.0 to 8.5 m.g.o., the time of the stationary period is longer as the velocity is larger.

The following table shows the uniformity in the magnetic properties which are obtained. The magnetic proprties of three pieces (cut from the center and the ends) of a 2 meter rod are given, the alloy being the same as mentioned in the previous examples.

Table Velocity 5 mnu/sec. stat. period 30 sec. End Center End Velocity 3 mrd/sec. stat. period 80 sec. End Center End Velocity 5 min/see. stat. period (30 sec. End Center End (BIDMX 8. 4 8.4 8. G Br. 13, 720 13,770 13, 640 Hin 77() 765 775 From part 1 of the table it will be seen that in any case the spread in the (BH)max values can be maintained within 0.5 m.g.o., which is acceptable for mass-production. lAs can be seen from parts 2 and 3 it ishowever also possible to obtain a more narrow spread e.g. 0.2-0.3 m.g.o. respectively. The spread in coercivity and remanence is generally within 1%. Thus, it is seen that magnet bodies having very uniform magnetic properties are obtained.

While I have described my invention in connection with specific examples and embodiments thereof I do not desire to be limited thereto, because it will be apparent to those skilled in this art that variations can be made in the composition of the material within the limits disclosed and in the details of the devices for carrying out the method according to the invention without departing from its spirit and scope of the invention.

What is claimed is:

1. In the manufacture of directionalgrain magnetically-anisotropic permanent magnets from an alloy of about 15 to 42% cobalt, about 10 to 20% nickel, about 6 to aluminium, and the remainder principally iron, the steps of forming a melt of the above alloy, closing with a starting rod one end of a mold defining a tubularshaped freezing zone having the cross section of the magnets, continuously feeding the melt to the open end of the mold, and moving the starting rod axially with a substantially constant speed of less than about 0.1 mm./sec. while withdrawing substantially all of the heat of solidification in the direction of movement to thereby progressively solidify the melt while in the solidication zone and a rod-shaped body is formed having cross-sectional dimensions for which, after magnetization, the rod has a (BI-I)max of at least 6.0 106 gauss-oersted.

2. In the manufacture of directional-grain magnetically-anisotropic permanent magnets from an alloy of about to 42% cobalt, about 10 to 20% nickel, about 6 to 10% aluminium, and the remainder principally iron, the steps of forming a melt of the above alloy, closing with a starting rod one end of a mold defining a tubularshaped freezing zone having the cross section of the magnets, continuously feeding the melt to the open end of the mold, moving the starting rod axially with a discontinuous rnotion in which the stationary periods are between 10 seconds and 200 seconds and the velocity is between about 0.1 and 10 mms/sec. and is smaller as the stationary periods are smaller, and withdrawing substantially all of the heat of solidification in an axial direction during the movement to thereby progressively solidify the melt while in the solidiiication zone, said rod being alternately moved at a velocity between 0.1 and 10 mm./sec. and held stationary for a period of time between 10 seconds and 200 seconds and which period of time is longer as the velocity is greater to thereby form a rod-shaped body having cross-sectional dimensions which, after magnetization, has an energy product of at least 6.() 106 gauss-oersted.

3. In the manufacture of directional-grain magnetically-anisotropic permanent magnets from an alloy of about 15 to 42% cobalt, about 10 to 20% nickel, about 6 to 10% aluminum, and the remainder principally iron, the steps of forming a melt of the above alloy, closing with a starting rod one end of a mold defining a tubular-shaped freezing zone having a diameter of about 14 mms., continuously feeding the melt to the open end of the mold, and moving the starting rod axially with a substantially constant speed of less than 0.1 mm./sec. while withdrawing all of the heat of solidification in the direction of movement to thereby progressively solidify the melt while in the solidification zone and a rod-shaped magnet body of said diameter is formed which, after magnetization, has a (BIDMX of at least 6.0 106 gauss-oersted.

References Cited by the Examiner UNITED STATES PATENTS 2,135,465 11/38 Eldred 22-200-1 2,578,407 12/51 Ebeling 22-212 FOREIGN PATENTS 531,774 10/ 56 Canada. 743,635 1/56 Great Britain.

MICHAEL V. BRINDISI, Primary Examiner. MARCUS U. LYONS, Examiner. 

1. IN THE MANUFACTURE OF DIRECTIONAL-GRAIN MAGNETICALLY-ANISOTROPIC PERMANENT MAGNETS FROM AN ALLOY OF ABOUT 15 TO 42% COBALT, ABOUT 10 TO 20% NICKLE, ABOUT 6 TO 10% ALUMINUM, AND THE REMAINDER PRINCIPALLY IRON, THE STEPS OF FORMING A MELT OF THE ABOVE ALLOY, CLOSING WITH A STARTING ROD ONE END OF A MOLD DEFINING A TUBULARA METALLURGICALLY BALLANCED MELT INTO A CASTING MOLD SURROUNDING THE RAIL ENDS IN A MANNER SUCH THAT THE STEEL FLOWS INTO THE MOLD ONLY ON BOTH SIDES OF THE UPPER CURVATURES OF THE RAIL HEADS, LATERALLY AND DOWNWARDLY PAST CATION IN THE DIRECTION OF MOVEMENT TO THEREBY PROGRESSIVELY SOLIDIFY THE MELT WHILE IN THE SOLIDIFICATION ZONE AND A ROD-SHAPED BODY IS FORMED HAVING CROSS-SECTIONAL DIMENNSIONS FOR WHICH, AFTER MAGNETIZATION, THE ROD HAS A (BH) MAX OF AT LEAST 6.0X10**6 GAUSS-OERSTED. 